Abstraction in Delphi

Question

Hello,

If I have child class,the child class inherits all methods from the parent,but how do I use functions from the child class in the parent class? Is that what abstraction is? How do I accomplish it?

My code:

type
cParent = class(TObject)
private
  function ParentFunction:byte;
  function ChildFunction:byte;virtual;abstract;
end;

type
cChild = class(cParent)
private function ChildFunction:byte;override;
end;

function cChild.ChildFunction:byte;
begin
  Exit(20);
end;

function cParent.ParentFunction:byte;
begin
  Exit(10);
end;

var
c:cParent;

begin
  c:= cParent.Create;
  WriteLn(c.ChildFunction);
  Readln;
end.

It compiles file,but I get abstraction exception.

Answer 1

Create the instance using the child class:

c:= cChild.Create;

Answer 2

c:= cParent.Create;
WriteLn(c.ChildFunction);

You create an instance of cParent class here. This class does only contain an abstract ChildFunction that can be overridden by other classes. The function is not implemented in cParent, so you get an abstract error.

The code works if you use the cChild class instead, where ChildFunction is implemented:

c:= cChild.Create;
WriteLn(c.ChildFunction);

For clarification, imagine a parent class named GeometricObject with an virtual abstract method CalculateVolume. You can now create child classes like SphereObject or BoxObject that implement CalculateVolume by using the formula for spheres/boxes. But it doesn't make sense to create an instance of GeometricObject and call CalculateVolume.

Answer 3

You create instance of cParent class. This class don't have implementation of childFunction. c must be instance of cChild class. Correct code: c := cChild.Create; WriteLn(c.ChildFunction);

Answer 4

It seems to me that you have confused inheritance relationships and possessive relationships. If you define a class "cChild" as inherited from class "cParent" then the class "cChild" is a "cParent", it does not mean, that the class "Parent" has access to any child classes. The only way you can call the abstract function "ChildFunction" in the class "cParent" is from another function, e.g. "ParentFunction", but only if the Object itself is not a "cParent":

function cParent.ParentFunction:byte;
begin
  Result := ChildFunction * 2;
end;

var
  c:cParent;

begin
  c:= cChild.Create;
  WriteLn(c.ParentFunction);
  Readln;
end.

Here, since c is a cChild, ParentFunction correctly calls ChildFunction, which is defined in cChild.

Answer 5

In response to schnaader, regarding CS 101:

"virtual" means the method can be overridden by child classes.

Not only this, but non-virtual functions may happily be overridden by child classes. The difference is very important: when a virtual function is called by the parent class, the CHILD function will be executed. This allows child classes to implement functions that will be called by parent classes without the parent ever knowing anything about them. This is the essence of polymorphism.